It is well known that the clarification or dewatering of sewage and industrial sludges and similar organic suspensions may be aided with the use of chemical reagents, added in order to induce a state of coagulation or flocculation which facilitates the process of solid/liquid or liquid/liquid separation from water. For this purpose, lime or salts of iron or aluminum have been utilized. More recently synthetic polyelectrolytes, particularly certain cationic copolymers of acrylamide, have been found to be of interest.
Cationically charged water soluble or water dispersible polymers are utilized in a variety of processes that involve the separation of solids or immiscible liquids dispersed or suspended in water from water, and the dewatering of solids containing water. These types of polymers, which may be natural or synthetic, are broadly termed coagulants and flocculants. These polymers can be utilized in such diverse processes as emulsion breaking, sludge dewatering, raw water clarification, drainage and retention aids in the manufacture of pulp and paper, flotation aids in mining processing and color removal. Polymers of this type generally work by neutralizing the anionic charge of the suspended solids, or liquids, which are to be removed.
It is known to supply and use water soluble, high cationic charge, low intrinsic viscosity (IV) coagulant polymers. Often they are provided in the form of aqueous solutions. These materials have relatively low IV and low molecular weight, which is sometimes an advantage. However, there are many occasions when it would be desirable if they could additionally perform in a manner that is typically associated with higher molecular weight materials.
Higher molecular weight, water soluble, polymers (generally of lower ionic charge) are frequently used as flocculants. Because of their higher IV and molecular weight, it is usually impracticable to supply them as aqueous solutions containing more than, at the most, 5% or 10% by weight of polymer. Even at 5% concentration the solutions are liable to have too high viscosity, and they may even be a rigid gel at higher concentrations. Accordingly flocculant polymers are generally supplied to the customer as powders or as reverse phase emulsions or dispersions in oil.
When the customer receives a powder, it is generally necessary for the customer to dissolve that powder in water prior to use, and the dissolution process can be slow and inconvenient. When the customer receives an emulsion, it is again generally necessary to dissolve the polymer of the emulsion into water during use and the resultant solution is contaminated with surfactant and the oil or other continuous phase of the emulsion. This is undesirable.
In order to avoid the disadvantages of dissolving powder or dealing with the oil continuous phase, there have been numerous attempts to provide water soluble, relatively high molecular weight, polymer in an aqueous composition, wherein the resultant composition has acceptable viscosity but much higher concentration than would be associated with that high molecular weight polymer if dissolved in water.
U.S. Pat. No. 6,001,920 discloses a pourable, liquid composition containing a blend of at least 8% water soluble high IV cationic polymer and a water soluble low IV cationic coagulant polymer which preferably comprises polyamine, and water soluble inorganic salt. According to the teaching of U.S. Pat. No. 6,001,920 it is necessary to include water soluble inorganic salt in the composition and the amount is normally at least 10% by weight and is usually at least 15% by weight of the composition, but it can be as much as 30% or even 35%. Preferably the concentration of salt is substantially the saturation concentration of that salt in the composition, preferably 90 to 100%, of the saturation concentration.
The preferred polyamines (cationic coagulant polymers) disclosed in U.S. Pat. No. 6,001,920 are copolymers of dimethylamine and epichlorohydrin. It is very typical that, when papermakers speak of “polyamines,” they are most often referring to a series of copolymers of dimethylamine and epichlorohydrin. The repeating unit of the linear form of the copolymer is —CH2—CHOH—CH2—N+(CH3)2—. The presence of a quaternary ammonium group within the backbone of this molecule ensures that it maintains its very strong cationic charge throughout the pH range of most papermaking operations. Molecular masses are typically between tens of thousands and hundreds of thousands of grams per mole.
Another typical polymer used as cationic coagulant polymers is polydiallyldimethylammonium chloride (DADMAC), which is a linear homopolymer formed from a monomer that has a quaternary ammonium and two unsaturated —CH═CH2 functionalities. The monomer itself is formed by reacting two equivalents of allyl chloride with dimethylamine. Free-radical polymerization of the “DADMAC” monomers yields a structure in which the quaternary ammonium groups are on rings that are included in the backbone of the polymer chain. This composition means that the poly-DADMAC macromolecules tend to be quite stiff, having a longer persistence length than, for instance, polyamines. For this reason, poly-DADMAC is expected to have a more extended conformation in solution. The molecular weight of DADMAC is typically in the range of hundreds of thousands of grams per mole, and even up to a million for some products.
EP 1522556A1 discloses a water-soluble polymer dispersions with fluidity and solubility properties so as to enable use in papermaking raw material pretreatments added to papermaking raw materials prior to machine operation. In particular, the water-soluble polymer dispersion is one comprising water-soluble polymer fine particles of 100 μm or less diameter having at least one ionic property selected from among cationic, amphoteric, nonionic and anionic properties together with a polyalkyleneimine wherein according to necessity an appropriate amount of water-soluble inorganic salt is incorporated. The polyalkyleneimine used is a preferably a polyethylenimine.
Polyethylene imine (PEI) is formed from monomer, which consists of a three-membered ring. Two corners of the monomer molecule consist of —CH2— linkages. The third corner is a secondary amine group, ═NH. In the presence of a catalyst this monomer is converted into a highly branched polymer with about 25% primary amine groups, 50% secondary amine groups, and 25% tertiary amine groups. This product is sometimes called “pure polyethyleneimine” in order to differentiate it from certain copolymers of ethyleneimine and acrylamide. The latter mixture is copolymerized to produce so-called “modified PEI,” that has a molecular mass up to about 2 million grams per mole.
The use and optimization of highly charged additives to a paper machine is never simple. There is a constant need to find new compositions for improving drainage, retention and formation in the paper making processes.